I'm imagining a system where you have two different lengths of time units. You can have a "scientific second" which is defined by distance and the speed of lig

I'm imagining a system where you have two different lengths of time units. You can have a "scientific second" which is defined by distance and the speed of light. Just like we have now in SI. When the rotation of the planet changes slightly you need to compensate by adding leap seconds every few years for things to sync up. Eventually they will need to add a leap second every day as the earth's rotation slows down. Some planet with high variation would be adding and removing these frequently. But, you can define a second as 1/86400th of a "day". For most civil purposes you just want to know when an event happened relative to the day/night cycle and the year. In which case the order of events is more important than the precise amount of time between the events. If precision is needed, you can always measure in scientific seconds and convert accordingly. I figure the "civil second" is fine for such a purpose because it's not like living beings can consistently and accurately keep track of time in their heads. Now for a planet with a rotation as slow as Venus the concept of a "day" is probably meaningless. It would be more like a season of light and darkness. So I suppose you'd just only use scientific seconds and count upwards?